Cleavage and exchange of major histocompatibility complex ligands employing azobenzene-containing peptides

ABSTRACT

In one aspect, the disclosure relates to major histocompatibility complex (MHC) molecules comprising a ligand in the peptide binding groove of the MHC molecule, whereby the ligand comprises an azobenzene (Abc), and at least two amino acid residues separated by the azo-group of the Abc, and wherein the amino acid residues are positioned to interact with the peptide binding groove of the MHC molecule. The disclosure also relates, among others, to means and methods for producing and using such MHC molecules, and the ligands therefor.

The invention relates to the field of major histocompatibility complex molecules (MHC). The invention in particular relates to an MHC molecule that contains a linker in the peptide binding groove of the MHC molecule, where the linker is cleavable thereby allowing for easy exchange with peptide antigens of interest. The invention further relates to means and methods for producing an MHC molecule having desired a MHC peptide in the peptide binding groove of the MHC and to cleavable ligands.

Chemical strategies have been progressively applied to understand and manipulate biological systems. The chemical reactivity of the employed reagents needs to be tuned such that interference with essential biochemical or cellular processes is prevented. Several bioorthogonal reactions have been developed to enable site-selective conjugation of macromolecules with a myriad of probes (e.g. luminescent dyes, photo-responsive moieties etc.)[1], yet the conditional breaking of bonds in the presence of a large heterogeneity of functional groups has received less attention. Cleavable linkers that can be chemoselectively addressed in a biocompatible manner have started to see deployment in disciplines such as biochemistry, proteomics, and cell biology.[2]

One successful application in immunobiology has facilitated the detection of disease-specific T cell responses within large reservoirs of other cells. T lymphocytes belong to the cellular arm of the adaptive immune system and are tasked to recognize and eliminate virus-infected or tumor cells. They express a large diversity of clonally distributed surface receptors that govern their specificity towards a cognate antigenic peptide fragment presented by major histocompatibility complexes (MHCs). Recombinantly produced oligomers of the latter heterotrimeric glycoprotein complex can bind to and stain T cells of corresponding specificity, and the conventional MHC tetramer format has become a cornerstone technology for mapping T cell responses in basic and clinical research on infectious diseases, autoimmunity, cancer and vaccine development.[3]

Libraries of MHC molecules such as tetramer libraries are among others accessible through synthetic ligands that are released through UV-induced cleavage of the peptide backbone, enabling a novel epitope to refill the evacuated MHC peptide-binding groove.[4] Arrays of the peptide-exchanged MHC tetramers enabled the interrogation of T cell repertoires regardless of their functional activity. Technical limitations such as low UV-penetration, variability in UV-irradiation, and the potential of photo- and thermal damage to the protein complexes, highlighted the need for alternative modes of cleavage. Chemoselective peptide exchange, although conceptually feasible, should avoid compromising the replacement epitope with its unprotected functionalities at the amino acid residue side-chains as well as N- and C-termini, or risk the loss of T cell antigen-recognition.

The present invention provides the use of azobenzene (Abc, Z) linkers that are sensitive to sodium dithionite (Na₂S₂O₄). The term Abc is in the description and claims directed towards the azobenzene structure. In the examples of the invention Abc typically relates to the azobenzene-containing linker. The Abc is a stereocenter-free building block that is accessible from readily available starting materials by a straightforward and cost-effective synthesis route. Furthermore, the Abc moiety is unaffected by reducing agents common to biological protocols (e.g. TCEP, DTT) and the fragmentation conditions have been demonstrated to be compatible with biomolecules and living systems.^([5])

SUMMARY OF THE INVENTION

In one aspect the invention provides a major histocompatibility complex (MHC) molecule comprising a ligand in the peptide binding groove of the MHC molecule, whereby said ligand comprises an azobenzene (Abc) wherein at least one of the aromatic rings comprises an electron-donor group. The electron-donor group is preferably a hydroxyl in the ortho position relative to the azo-group, the azobenzene further comprises at least two amino acid residues separated by the azo-group of the Abc, and wherein the amino acid residues are positioned to interact with the peptide binding groove of the MHC molecule. It is preferred that said ligand is an MHC peptide antigen of which amino acid residues that are located between the amino acid residues have been replaced by an Abc. The Abc preferably comprises the general formula I

wherein at least one of the aromatic rings comprise an electron-donating group; M is independently C, S, N or O; Z1 and Z4 each comprise an amino acid residue positioned to interact with the peptide binding groove of the MHC molecule; Z2 and Z3 are independently H, hydroxyl, carboxy, keto, or a linear or branched C₁-C₁₀ alkyl, optionally substituted with an oxy, hydroxyl, nitrogen, nitroxy, sulhydryl or sulfide group.

In a preferred embodiment the Abc comprises the general formula II

The ligand preferably comprises the general formula III

wherein,

A, B, C, D, X and Y are each independently an amino acid residue;

n₁, n₂, n₃ and n₄ are each independently 0-11; and

-   -   n₁+n₂+n₃+n₄ equals 2-18.

The invention further provides a complex comprising one, two or more MHC molecules of the invention.

Further provided is a composition comprising an MHC molecule of the invention and/or a complex of the invention and an MHC peptide antigen.

The invention further provides a method of producing an MHC molecule comprising

producing an MHC molecule of the invention;

contacting the produced MHC molecule with a reducing agent; and

contacting said MHC molecule with an MHC peptide antigen.

The invention further provides a method of detecting an MHC molecule comprising producing an MHC molecule by a method of the invention, and detecting the MHC molecule. The MHC molecule, the peptide in the peptide binding groove of the MHC molecule or both preferably comprise a label.

The invention further provides a solid surface comprising an MHC molecule or a complex of the invention.

The invention further provides an azobenzene of formula I

wherein at least one of the aromatic rings comprise an electron-donating group; M is independently C, S, N or O; Z1 and Z4 each comprise an amino acid residue positioned to interact with the peptide binding groove of an MHC molecule; Z2 and Z3 are independently H, hydroxyl, carboxy, keto, or a linear or branched C₁-C₁₀ alkyl, optionally substituted with an oxy, hydroxyl, nitrogen, nitroxy, sulhydryl or sulfide group.

Further provided is an azobenzene of the invention, for use in the production of an MHC-molecule comprising a peptide in the peptide binding groove of the MHC-molecule.

DETAILED DESCRIPTION OF THE INVENTION

As used herein the term “polypeptide” refers to a molecule comprising at least 50 amino acids or functional equivalents thereof that are linked to each other via peptide bonds. In its unfolded state, the polypeptide is typically a linear molecule but can be (partly) circular. A peptide typically contains between 2 and 49 amino acids that are linked to each other via peptide bonds.

An amino acid can be a natural or synthetic amino acid such as for instance an alpha, beta, or gamma or higher (omega) amino acid, i.e. including 1, 2, 3, or more carbon spacings between amino groups and carboxylic acids. An amino acid (chain) can be a natural amino acid (chain) or a synthesized amino acid (chain) or a combination thereof. A peptide is a natural peptide or a synthesized peptide or a combination thereof. In its unfolded state a peptide is typically linear, but can be (partly) circular. A peptide typically does not have a dominant tertiary structure. It typically accommodates a range of tertiary structures. A peptide as used in the invention is typically easily dissolvable in diverse solvents. Such solvents are for instance physiological solutions, such as a physiological sodium chloride solution. Alternatively, peptides can be dissolved in a solvent as DMSO and subsequently brought into an aqueous environment.

The terms “peptide antigen”, “MHC peptide antigen” are used interchangeably herein and refer to an MHC ligand that can bind in the peptide binding groove of an MHC molecule. The peptide antigen can typically be presented by the MHC molecule. A peptide antigen typically has between 8 and 25 amino acids that are linked via peptide bonds. The peptide can contain modification such as but not limited to, the side chains of the amino acid residues; the presence of a label or tag; the presence of a synthetic amino acid, a functional equivalent of an amino acid or the like. Typical modifications include those as produced by the cellular machinery, such as glycan addition and phosphorylation. However, other types of modification are also within the scope of the invention.

A functional equivalent of an amino acid is a molecule that can replace one or more amino acids in an amino acid chain. The functional equivalent is preferably capable of forming bonds with amino acids in two separate positions such that it can form an internal part of a (poly)peptide or peptidomimetic chain. The functional equivalent does not have to have a natural counterpart. Such a functional equivalent can be incorporated into a peptide or peptide antigen of the invention.

The major histocompatibility complex (MHC) is a set of cell surface molecules encoded by a large gene family in all vertebrates. In humans, MHC is also called human leukocyte antigen (HLA).

An MHC molecule displays a peptide and presents it to the immune system of the vertebrate. The peptide also referred to as ligand, peptide antigen or MHC peptide antigen can be either a self or a non-self peptide. MHC-class I molecules typically present the peptide antigen to CD8 positive T-cells whereas MHC-class II molecules present the peptide antigen to CD4 positive T-cells.

MHC molecules are encoded by polygenic and exceptionally polymorphic gene families. It is thought that the diversity provides a survival advantage against pathogens. Allelic polymorphism for each of the genes is particularly prominent in those amino acid residues that line the peptide-binding groove of these molecules. The observed diversity in the amino acid residues of the peptide binding groove of the MHC molecules defines the peptide-binding and the presentation repertoire of the individual MHC-molecule (Chang et al 2011; Frontiers in Bioscience, Landmark Edition, Vol. 16: 3014-3035). Through the vast repertoire of allelic variants of MHC molecules such as the HLA molecules in the general population, each of them capable of binding a distinct set of peptide antigens, a mechanism is created to deal with the large diversity of antigens of pathogens. At the same time, significant cross-reactivity in peptide antigen binding to different MHC/HLA molecules has been observed. It has been proposed to cluster HLAs that bind overlapping collections of peptides into supertypes. For HLA, the various HLA-A and HLA-B molecules have been grouped into a limited number of supertypes based on their ability to binding similar peptide sequences (Sidney et al 2008; BMC immunology Vol 9:1). Crystallography and experimental evidence has revealed that peptide binding specificity is primarily governed by the physiochemical properties of the B and F binding pockets in a coupled fashion (see FIG. 1 of Chang et al 2011 supra). The B and F binding pockets typically bind to so-called “anchor residues” in the peptide that define the binding of the peptide in the peptide binding groove of the MHC. The specificity of the pockets for anchor residues has been elucidated for a large number MHC-molecules. For HLA the pocket specificity is among others described in Sidney et al (2008 supra) which is incorporated by reference herein for the binding specificity of the B and F pockets for the respective HLA molecules and HLA supertypes mentioned therein.

The ligand that binds to the peptide binding groove of the MHC molecule can be a naturally occurring peptide but can also be synthetically created using the knowledge of the binding specificity of the B and F pocket of the particular MHC molecule or the supertype family it belongs to.

The ligand of the present invention utilizes an azobenzene as a cleavable linker. The azobenzene is a chemical compound composed of two phenyl rings linked by a N═N double bond (azo group). It is the simplest example of an azo compound. The term ‘azobenzene’ or simply ‘azo’ is often used to refer to a wide class of molecules that share the core azobenzene structure, with different chemical functional groups extending from the phenyl rings. Azo compounds are sometimes referred to as ‘diazenes’. In the present invention the azobenzene is preferably sensitive to sodium dithionite (Na₂S₂O₄).

At least one of the aromatic rings of the azobenzene comprises an electron-donating (or electron-donor) group. The electron-donor group is preferably is preferably an amine group, an amide group, an aromatic group, an alkene group; an alkoxy group, a hydroxyl group or a ketone or a carboxyl group. The electron-donor group is preferably in the ortho or the para (mesomeric) position relative to the position of the azo-group. In one aspect the amine is a primary, secondary or tertiary amine. In a preferred embodiment the amino is a primary amine. In another aspect the electron-donor group is a hydroxyl, ketone, or carboxy-group. In a preferred embodiment the electron-donor group is a hydroxyl group. The hydroxyl, ketone or carboxy-group is preferably in the ortho-position relative to position of the azo-group. The alkoxy group, when present, is preferably in the para-position relative to the azo-group. In a preferred embodiment the azobenzene of the invention comprises the general formula IV,

wherein

“*” indicates that the azobenzene has at least one hydroxyl in the ortho position relative to the azo-group; The hydroxyl is preferably in the position *1 as indicated in the general formula IV.

M is independently C, S, N or O;

Z1 and Z4 each comprise an amino acid residue positioned to interact with the peptide binding groove of an MHC molecule;

Z2 and Z3 are independently H, hydroxyl, carboxy, keto, or a linear or branched C₁-C₁₀ alkyl, optionally substituted with an oxy, hydroxyl, nitrogen, nitroxy, sulhydryl or sulfide group.

The azobenzene can obtained with the use of readily available starting materials by a straightforward and cost-effective synthesis route. Furthermore, the Abc moiety is unaffected by reducing agents common to biological protocols (e.g. TCEP, DTT) and the correct fragmentation conditions have been demonstrated to be compatible with biomolecules and living systems.[5]

The ligand further comprises amino acid residues separated by the azo group of the Abc and positioned to interact with peptide binding groove of the MHC molecule. The separation by the azo group ensures that upon cleavage of the azo group, the ligand is fragmented into fragments that each contain less amino acid residues interacting with the peptide binding groove.

Suitable ligands can be generated using the available 3D structures of MHC complexes and the knowledge on the binding pocket specificity of the respective MHC molecules. Binding characteristics can be evaluated using 3D-crystallography as exemplified in the examples. A suitable starting point for the design of the ligand is a known MHC peptide antigen. One or more of the amino acids can be replaced by the Abc. In a preferred embodiment one or more of the amino acid residues that are located between anchor amino acid residues are replaced by the Abc. In a preferred embodiment the ligand is an MHC-peptide antigen of which amino acid residues that are located between anchor amino acid residues have been replaced by the Abc.

The Abc preferably comprises the general formula I

wherein

at least one of the aromatic rings comprise an electron-donating group;

M is independently C, S, N or O;

Z1 and Z4 each comprise an amino acid residue positioned to interact with the peptide binding groove of the MHC molecule;

Z2 and Z3 are independently H, hydroxyl, carboxy, keto, or a linear or branched C₁-C₁₀ alkyl, optionally substituted with an oxy, hydroxyl, nitrogen, nitroxy, sulhydryl or sulfide group.

Although phenyl is the preferred aromatic group, i.e., M═C, other heteroaromatic groups may be used, e.g., pyridyl, M═N. X may also be S or O. Z2 and Z3 indicate that the present activity of the cleavable linker is maintained with certain modifications to the aryl diazo structure. Tolerable substitutions include lower alkyl, hydroxyl, carboxy or keto.

Z1 and Z4 each comprise an amino acid residue positioned to interact with the peptide binding groove of the MHC molecule. The amino acid residue is preferably positioned by C₁₋₂ alkyl group that is located between the benzene ring and the amino acid residue. Z1 or Z4 preferably further comprises an NH located between the alkyl group and the amino acid residue. The C₁₋₂ alkyl group, preferably of Z4, may optionally be substituted by keto group.

Z1 is preferably linked to the phenyl at the meta position relative to the azo group, preferably at the meta position indicated by M, in general formula I. Z4 is preferably linked to the phenyl at the para position relative to the azo group.

The Abc preferably comprises the general formula II

The left and right “—” line indicates that the Abc is linked to an amino acid residue at that position. It does not indicate the presence of a “—CH3” group at that position.

The ligand preferably comprises the general formula III

wherein,

A, B, C, D, X and Y are each independently an amino acid residue;

n₁, n₂, n₃ and n₄ are each independently 0-11; and

n₁+n₂+n₃+n₄ equals 2-18.

The Abc is preferably a trans-Abc.

The integers n₂ and n₃ are preferably chosen such that the amino acid residues X and Y are positioned to interact with the peptide binding groove of the MHC molecule.

MHC class I molecules typically bind peptides that are 8-10 amino acid residues in length. For MHC class I molecules n₁+n₂+n₃+n₄ equals 2, 3, 4, 5, 6, 7 or 8. n₁+n₂+n₃+n₄ preferably equals 2, 3, 4, 5, or 6; preferably 2, 3, 4 or 5; more preferably 2, 3 or 4. In a preferred embodiment n₁+n₂+n₃+n₄ equals 2 or 3 for an MHC class I ligand. The structure between B and C extends in essence same distance as 4 amino residues. As MHC class I peptides are typically 8-10 amino acids the ligand of formula III typically comprise 4-6 amino acids.

Because the antigen-binding groove of MHC class II molecules is open at both ends while the corresponding groove on class I molecules is closed at each end, the antigens presented by MHC class II molecules can generally be longer. MHC class II binding peptides are typically 15-24 amino acid residues long. Artificial class II binding peptides can be smaller than 15 amino acid residues. Accommodating the size of the Abc the sum of n₁+n₂+n₃+n₄ for typical MHC II ligands is 2-18. The lower end of the sum range is preferably 3. In a preferred embodiment the sum is 6-14; more preferably 7-13.

The MHC class II binding groove typically has 4 major pockets. These pockets accommodate the side chains of residues 1, 4, 6 and 9 of the 9-mer core region of the binding peptide. This core region largely determines binding affinity and specificity (Wang et al 2008: PLoS Comput Biol 4(4): e1000048. Doi:10.1371/journal.pcbi.100048). Structural features of binding of peptides to the respective grooves can be found among other in Rammensee, H.-G. (1995. Chemistry of peptides associated with MHC class I and class II molecules. Curr. Opin. Immunol. 7:85). Amino acid residues that bind to the specific pockets of the peptide binding groove of MHC molecules are also referred to as anchor residues. Amino acid residues that are positioned to interact with the peptide binding groove in a ligand of the invention are preferably anchor amino acid residues.

For MHC class I the amino acid residues are preferably positioned to interact with the peptide binding groove of the MHC molecule at the B and F pockets of the binding groove of an MHC I molecule. The ligand is preferably a ligand as depicted in table S1, where Z is preferably the Abc of formula II. In another preferred embodiment the ligand is a ligand as depicted in FIG. 1. For MHC-I ligands n₂ or n₃ or both are preferably independently 0 or 1. In a preferred embodiment n₂ or n₃ or both are preferably 1.

For MHC class II the amino acid residues are preferably positioned to interact with at least two of the major pockets of the MHC class II peptide binding groove. The ligand preferably contains the amino acid residues of the core region at position 1 and 9. The ligand is preferably a ligand as depicted in table S7 where Abc is preferably the Abc of formula II. For MHC-II ligands n₂ or n₃ or both are preferably independently 0, 1, 2 or 3. In a preferred embodiment n₂+n₃ is preferably 3 or 4. Preferably n₂+n₃ is 3.

The MHC-molecule can be an MHC class I, MHC class II, a non-classical MHC molecule or a functional part, derivative and/or analogue thereof. MHC II peptides with light sensitive conjugates have been produced among others in Grotenberg et al (2007: The J. of Biol. Chem. Vol 282, pp. 21425-21436). In a preferred embodiment the MHC molecule is an MHC I molecule. Preferably the MHC molecule is an HLA molecule. Preferably said MHC molecule is a soluble MHC-molecule, preferably as described in Garboczi D N, Hung D T, Wiley D C. HLA-A2-peptide complexes: refolding and crystallization of molecules expressed in Escherichia coli and complexed with single antigenic peptides. Proc Natl Acad Sci USA. 1992 Apr. 15;89(8):3429-33. The MHC molecule is preferably a human MHC molecule.

A functional derivative of an MHC-molecule is a molecule that is not derived from nature, but that shares at least a peptide binding property with an MHC molecule in kind, not necessarily in amount. For instance, modified MHC-molecules comprising one or more amino acid differences with natural MHC molecules but that retain a peptide binding function are functional derivatives in the context of the present invention. Similarly, molecules comprising (part of) peptide binding domains from two or more MHC molecules and that are capable of binding a peptide are also considered functional derivatives. Modifications that are typically tolerated are those that are not in the peptide binding domains. Other mutations or modifications that are tolerated are in the variable domains of the peptide binding domains of MHC molecules. Such modifications typically alter the binding specificity of the MHC molecule (i.e. which peptide is bound). Such modifications are therefore also considered functional derivatives of MHC molecules of the invention.

Several molecules share the peptide binding properties of MHC-molecules but have evolved to serve a different purpose in the cell. Such molecules are considered functional analogues of an MHC molecule of the present invention. Domains that are involved in (poly)peptide binding can be combined with such domains from MHC molecules. MHC-molecules or functional parts, derivatives and/or analogues thereof may further contain other parts that are not normally associated with MHC molecules. Such other parts may, for instance, comprise labels, tags, association and/or multimerisation domains and other elements.

The technology of the present invention can be used to specifically destabilize ligands bound to MHC molecules, or to functional parts, derivatives and/or analogues thereof. Destabilization of the MHC bound ligands then results in the generation of ligand-free MHC molecules without exposure to harsh conditions. The resulting ligand-free MHC molecules may then be used either in the ligand-free form or may be loaded with one or multiple ligands, peptide antigens of choice.

Thus in a preferred aspect of the present invention, an MHC molecule or a functional part, derivative and/or analogue thereof, comprises a peptide antigen (also referred to as ligand) in the peptide-binding groove of said MHC molecule or a functional part, derivative and/or analogue thereof. The Abc is preferably present in the peptide antigen as this warrants release of the peptide antigen from the otherwise unmodified MHC molecule or a functional part, derivative and/or analogue thereof. The resultant ligand-free MHC molecules may be used directly or be loaded with one or more other ligands, peptide antigens. To this end the invention further provides a composition comprising an MHC molecule of the invention. Such a composition can be provided with a peptide antigen to be loaded onto the MHC-molecule. Thus, further provided is a major histocompatibility complex (MHC) molecule or a functional part, derivative and/or analogue thereof, comprising a peptide antigen in the peptide-binding groove of said molecule and wherein said peptide antigen comprises said Abc. The composition can also comprise a further peptide. In a preferred embodiment said further peptide is a peptide antigen capable of binding in the peptide-binding groove of said MHC molecule, i.e. a ligand for said MHC-molecule. The further peptide may be added after the exposure to the reducing agent. It is preferred that the further peptide is present when the reducing agent is added to the composition. The peptide can take the place of the leaving fragments. The resultant MHC-molecule or functional part, derivative and/or analogue thereof is thereby loaded with the further peptide. Thus the composition contains the newly loaded MHC-molecule (or functional part, derivative and/or analogue thereof) and fragments of the leaving peptide.

In another aspect the invention provides a method for producing an MHC molecule or a functional part, derivative and/or analogue thereof, or a MHC-molecule complex comprising a further peptide, comprising producing an MHC molecule of the invention, contacting the produced MHC molecule with a reducing agent and contacting said MHC molecule with an MHC peptide antigen. The MHC molecule of the invention is preferably an MHC molecule comprising a ligand in the peptide binding groove of the MHC molecule, whereby said ligand comprises an azobenzene (Abc) wherein at least one of the aromatic rings comprises an electron-donor group. The electron-donor group is preferably a hydroxyl in the ortho position relative to the azo-group. The azobenzene further comprises at least two amino acid residues separated by the azo-group of the Abc, and wherein the amino acid residues are positioned to interact with the peptide binding groove of the MHC molecule.

The reducing agent cleaves the Abc into smaller fragments. This allows the easy removal of the leaving peptide (or fragments thereof) from the MHC-molecule or functional part, derivative and/or analogue thereof. Removal does not require harsh conditions and thus not or only minimally interfere with the activity of the molecule. The free MHC-molecule can be provided with a desired peptide. Using a method of the invention it is possible to produce large amounts of MHC-molecule having the leaving peptide. This preparation can subsequently be used to generate MHC-molecules comprising a variety of different ligands (antigenic peptides) with a method of the invention. The contacting with the reducing agent and the contacting with the desired peptide can be performed in one step.

The invention further provides a method of detecting an MHC-molecule comprising producing an MHC molecule according to a method of the invention and detecting the MHC-molecule or the peptide in the peptide binding groove of the MHC molecule. This aspect is for example useful for diagnostic purposes. Binding can be detected in various ways, for instance via T-cell receptor or antibody specific for said peptide presented in the context of said MHC-molecule. Binding is preferably detected by detecting a label that is associated with said peptide or said MHC-molecule. Labelling of the peptide can be done by tagging said peptide with a specific binding molecule such as biotin that can subsequently be visualized via for instance, labelled streptavidin or analogues thereof. In a preferred embodiment said peptide comprises said label. In this way any peptide bound to said MHC-molecule can be detected directly. Detection of binding is preferably done for screening purposes, preferably in a high throughput setting. Preferred screening purposes are screening for compounds that affect the binding of said peptide to said MHC-molecule. For instance, test peptides or small molecules can compete with binding of said peptide to said MHC-molecule. Competition can be detected by detecting decreased binding of said peptide. A preferred method for detecting binding of said peptide to said MHC-molecule is measured by means of fluorescence anisotropy. In this way manipulations of the sample wherein said binding is performed can be reduced. Reduction of sample manipulations is a desired property for high throughput settings. Other preferred means for detecting binding of said peptide are monitoring radioactivity or by monitoring binding of an MHC conformation dependent binding body, preferably an antibody or a functional part, derivative and/or analogue thereof. Other preferred means include the use of a T-cell receptor specific for the combination of said peptide, MHC molecule. In a preferred embodiment inhibition or enhancement of binding of said peptide to said MHC-molecule is measured. In a preferred embodiment said method is used for determining binding of said desired peptide in the presence of a test or reference compound.

The invention further provides an MHC molecule obtainable by a method of the invention. Further, the invention provides a composition comprising an MHC-molecule according to the invention, wherein said composition comprises an MHC-molecule comprising a peptide comprising an Abc and an MHC-molecule comprising a further peptide.

The invention further provides a complex comprising at least two MHC molecules of the invention. A complex comprising at least two MHC molecules of the invention is preferably a dimer, a trimer, a tetramer, a pentamer or a dextramer of MHC-molecules. In a preferred embodiment the complex is a tetramer. The term “complex” as used herein refers to a protein complex wherein two or more MHC molecules are physically linked to each other and are functional. The term does not refer to structures as inclusion bodies or precipitates consisting essentially of denatured or otherwise non-functional MHC molecules. The term complex typically refers to a multimer of two or more MHC molecules that are in solution. Association of two or more MHC molecules via a solid surface is typically not referred to as a complex but as a solid surface. MHC molecules can also be associated to each other by coupling them to, for instance, a polymer. Such associations are also captured under the term complex, unless the polymer is in the form of a gel or other solid surface. In the latter case the association is referred to as a solid surface comprising two or more MHC-molecules. A solid surface can comprises a complex of the invention as also indicated herein below. A preferred complex is an MHC tetramer. Complexes such as dimers, trimers, tetramers and the like have a higher affinity for the particles and cells carrying T-cell receptors than the single MHC molecule. Such complexes are therefore important tools in the analysis of T-cell populations. The invention thus further provides a complex comprising at least two MHC molecules of the invention. Means and methods for producing complexes containing two, three, four and five MHC-molecules or functional parts, derivatives and/or analogues thereof are available in the art. Thus the present invention further provides a complex comprising two, three, four or five MHC-molecules of the invention or functional parts, derivatives and/or analogues thereof. In a preferred embodiment, said complexes comprise MHC molecules having the same T-cell receptor specificity. However, this need not always be the case. Considering the relative ease with which MHC molecules can be provided with different peptides using a method of the invention, complexes comprising two or more T-cell receptor specificities are within the scope of the present invention. The invention further provides a solid surface comprising at least two MHC molecules or complexes of the invention. In a preferred embodiment, said solid surface is provided with a complex of the invention, preferably a complex comprising a single peptide, or multiple peptides associated with the same disease or pathogen. The solid surface can be a bead or a microchip. The solid surface can be any solid material. The solid surface is preferably a biochemically inert surface such as a glass, plastic or metallic surface. The surface can also be a polymer surface, such as a gel. The solid surface is typically essentially two dimensional. However, three dimensional surfaces such as gels are within the scope of the invention. The surface may have undergone pre-treatment prior to coating of the MHC molecule, composition or complex of the invention. Such pre-treatment may include but is not limited to polyacrylamide film-coating as described by Soen et al (PLoS biology; 2003: vol 1, page 429-438). The invention further provides a microarray comprising an MHC molecule, composition or complex of the invention. Means and methods for producing a (micro)array comprising an MHC-molecule complex coupled to antigenic peptide is described by Soen et al mentioned above. The artisan is referred to said reference for guidance as to the generation of a (micro)array of the invention.

The invention further provides a composition comprising an MHC molecule of the invention and/or a complex of the invention and an MHC peptide antigen.

The invention further provides a method of producing an MHC molecule comprising

producing an MHC molecule of the invention;

contacting the produced MHC molecule with a reducing agent; and

contacting said MHC molecule with an MHC peptide antigen. The two contacting step are preferably performed by providing a sample comprising the MHC-molecule with the MHC-peptide antigen and the reducing agent. It is preferred that the MHC peptide antigen is present when the reducing agent is added. An MHC-molecule that does not contain a peptide in the peptide binding groove can be unstable under certain conditions. To avoid such it is preferred that the MHC-peptide antigen is added prior to addition of the reducing agent. Preferably one MHC-peptide antigen is added per reaction, but this is not essential. It is within the scope of the invention to add more than one different MHC peptide antigen per reaction.

The reducing agent can be any agent capably of reducing the azo group in an Abc of the invention. A preferred reducing agent is dithionite. Preferably sodium dithionite. Other reducing agents can also be used. It is preferred that the reducing agent is biocompatible. A non-limiting example is SnCl₂ using 0.1 M HCl. Dithionite is preferred as it is milder than the indicated SnCl₂ treatment. The artisan appreciates that the reducing agent can be varied depending on the electron-donor group and/or the position of the electron-donor group relative to the azo-group.

The invention further provides a method of detecting an MHC molecule comprising producing an MHC molecule of the invention and detecting the MHC molecule. In a preferred embodiment the MHC molecule, a peptide in the peptide binding groove of the MHC molecule or both comprise a label.

The invention further provides a solid surface comprising an MHC molecule or complex of the invention.

The invention further provides an azobenzene of formula I

wherein

at least one of the aromatic rings comprise an electron-donating group;

M is independently C, S, N or O;

Z1 and Z4 each comprise an amino acid residue positioned to interact with the peptide binding groove of an MHC molecule;

Z2 and Z3 are independently H, hydroxyl, carboxy, keto, or a linear or branched C₁-C₁₀ alkyl, optionally substituted with an oxy, hydroxyl, nitrogen, nitroxy, sulhydryl or sulfide group.

An azobenzene of the invention preferably comprises a structure of formula IV

wherein

“*” indicates that the azobenzene has at least one hydroxyl in the ortho position relative to the azo-group; The hydroxyl is preferably in the position *1 as indicated in the general formula IV.

M is independently C, S, N or O;

Z1 and Z4 each comprise an amino acid residue positioned to interact with the peptide binding groove of an MHC molecule;

Z2 and Z3 are independently H, hydroxyl, carboxy, keto, or a linear or branched C₁-C₁₀ alkyl, optionally substituted with an oxy, hydroxyl, nitrogen, nitroxy, sulhydryl or sulfide group.

The “*” indicates the four ortho positions for the at least one hydroxyl. Not all four positions indicated by the “*” have to contain a hydroxyl. Only one of the four positions needs to contain a hydroxyl. In a preferred embodiment the Abc contains one hydroxyl in the ortho position relative to the azo group. In a preferred embodiment the hydroxyl group is in the position *1 as indicated in the general formula IV. The preferred position of the hydroxyl indicated by “*1” is also the preferred position for the hydroxyl group in the azobenzene of the invention such as but not limited to the azobenzene of formula's I, II and III.

In the structural formula's the indicators “*”, “M”, “Z1-Z4”, “A-D”, “X”, “Y”, “n1-n4” and other indicators have the same meaning. So where in the description an indicator is defined such definition applies the same for the indicator in any of the respective formula's. The same holds for the C₁₋₂ alkyl group between a benzene ring and an amino acid residue.

The invention further provides an azobenzene of the invention for use in the production of an MHC-molecule comprising a peptide in the peptide binding groove of the MHC-molecule.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Design of HLA-A*11:01-restricted Abc ligands. a) Replacement of four amino acids residues (11 bond lengths) with an azobenzene-containing (Abc) tetrapeptide isostere (12 bond lengths). b) MHC stability ELISA of UV-sensitive A*11:01 molecules peptide-exchanged with A*11:01-restricted epitope (1), A*02:01-restricted epitope (2) and A*11:01-restricted Abc ligands (3 to 6) upon UV irradiation. MHC molecules before (−UV) and after (+UV) UV irradiation in the absence of rescue peptides were included as controls. c) Sequences of the epitopes (1 and 2) and newly synthesized Abc ligands (3 to 6) that were used in (b). The position of the Abc moiety in the parent peptides is indicated in red. Anchor residues of the peptides are underlined. d) Overlay of crystal structures of 4 (cyan) (PDB reference ID: 4BEO, this work) and its parent peptide (yellow) (PDB reference ID: 2HN7) in an A*11:01 molecule (grey).

FIG. 2. Cleavage kinetics and conditions of HLA-A*11:01-restricted Abc ligand. a) Cleavage of 4 resulted in two aniline products (7 and 8) upon addition of sodium dithionite. Reaction was confirmed using LC-MS. b) 4 is incubated in the presence of 9 at 1:1 ratio with 1, 2.5 or 5 mM of sodium dithionite and the reactions were quenched after 1, 2 or 5 minutes. The reaction mixtures were analyzed for the presence of intact ABC ligands using LC/MS. c) Refolded A*11:01 molecules bearing 4 were peptide-exchanged were with A*11:01-restricted peptides (1 and 10) and A*02:01-restricted peptide (2) in the presence of 5 to 20 mM sodium dithionite. Controls with no peptides added (−) to the MHC molecules were included. Stable MHC molecules were quantified using MHC stability ELISA. d) Freshly isolated PBMCs were incubated with 10 mM (middle column) and 100 mM (right column) sodium dithionite for 1 hour (top row) or 16 hours (bottom row) to assess cellular toxicity of sodium dithionite. Cells were stained with anti-CD8 antibodies, Annexin V and LIVE/DEAD® viability dye, and analyzed on flow cytometry. Plots shown were gated on CD8⁺ cells. Numbers in each plot are cells expressed as a percentage of total CD8⁺ population.

FIG. 3. Detection of antigen-specific CD8⁺ T cells using A*11:01 MHC tetramers generated from UV-mediated peptide exchange or sodium dithionite-mediated peptide exchange. a) Schematic diagram of experimental workflow. Freshly isolated PBMCs from an A*11:01-positive volunteer were stimulated with A*11:01-restricted epitopes EBV BRLF1₁₃₄₋₁₄₂ (1, upper row) and Influenza A MP₁₃₋₂₁ (10, lower row) and clonally expanded for 14 days. Antigen-specific CD8⁺ T cells were then labeled with cognate peptide-bound A*11:01 MHC tetramers (red) and detected via flow cytometry. b) A*11:01-EBV BRLF1₁₃₄₋₁₄₂ (columns 1 and 4), A*11:01-Influenza A MP₁₃₋₂₁ (columns 2 and 5) and A*11:01-Abc (column 3) tetramers were incubated with the PBMCs to detect EBV BRLF1₁₃₄₋₁₄₂ (1, upper row) and Influenza A MP₁₃₋₂₁ (10, lower row) -specific CD8⁺ T cells 14 days post-stimulation. Number in each plot represents tetramer-positive cells as a percentage of total CD8⁺ cell population. Plots in columns 1 to 2 and columns 3 to 5 are tetramer staining performed using UV-derived and Abc-derived tetramers respectively.

FIG. 4. Effects of reductive and oxidative cleavage conditions on epitopes of interest. Mass spectrometry analysis of peptides Influenza A MP₁₃₋₂₁ (10, left column) and EBV BMLF-1₂₅₉₋₂₆₇ (11, right column) after incubation with PBS, 10 mM sodium dithionite or 0.3 mM sodium periodate. 10 and 11 remained unmodified when incubated in PBS or 10 mM sodium dithionite. The N-terminal serine of 10 was cleaved when incubated in 0.3 mM sodium periodate. Incubation with 0.3 mM sodium periodate resulted in oxidation of cysteine and methionine residues of 11. Unmodified and modified residues are shown in blue and red respectively.

FIG. 5. (figure S1)

Mass spectrometry (MS) analysis of the synthesized Abc ligands by LC-MS IT-TOF. The peptides were first separated by liquid chromatography (LC) on a C18 column prior to the measurement of its mass to charge ratios (m/z). Details of the Abc ligands are shown in Table S1. The reference to −Z-indicates Z the presence of an Abc of formula II

A) Conditional ligand: AIF-Z-TK; Empirical formula: C43H59N9O9;

Exact Mass: 845.44; Molecular Weight: 845.98

Peak #1; Retention Time: 11.520 min; Base Peak m/z: 846.3542;

Base Peak Intensity: 937599; Polarity: Pos

B) Conditional ligand: AIM-Z-YPK; Empirical formula: C49H68N10O10S;

Exact Mass: 988.48; Molecular Weight: 989.19

Peak #2; Retention Time: 9.033 min; Base Peak m/z: 495.2475;

Base Peak Intensity: 1413471; Polarity: Pos

C) Conditional ligand: QVPL-Z-YK; Empirical formula: C51H71N11O11;

Exact Mass: 1013.53; Molecular Weight: 1014.19

+ESI Scan; 15 scans: 6.088-6323 min Frag=180.0V; Polarity: Pos D) Conditional ligand: KTF-Z-PK; Empirical formula: C45H62N10O9; Exact Mass: 886.47; Mol. Wt.: 887.04 Peak #: 1; Retention Time: 10.733 min; Base Peak m/z: 887.3648;

Base Peak Intensity: 658640; Polarity: Pos

E) Conditional ligand: FLPS-Z-SV; Empirical formula: C46H61N9O11;

Exact Mass: 915.45; Molecular Weight: 916.05

Peak #: 2; Retention Time: 12.513 min; Base Peak m/z: 916.4301;

Base Peak Intensity: 6800116; Polarity: Pos

F) Conditional ligand: LLF-Z-YV; Empirical formula: C50H64N8O9;

Exact Mass: 920.48; Molecular Weight: 921.11

Peak #: 2; Retention Time: 14.293 min; Base Peak m/z: 921.4783;

Base Peak Intensity: 2512246; Polarity: Pos, Event

G) Conditional ligand: NLVP-Z-TV; Empirical formula: C44H64N10O11;

Exact Mass: 908.48; Molecular Weight: 909.05

Peak #: 2; Retention Time: 12.793 min; Base Peak m/z: 909.4549;

Base Peak Intensity: 1161661; Polarity: Pos

H) Conditional ligand: NLVP-Z-VATV; Empirical formula: C52H78N12O13;

Exact Mass: 1078.58; Molecular Weight: 1079.27

Peak #: 2; Retention Time: 13.300 min; Base Peak m/z: 1079.5823;

Base Peak Intensity: 1070704; Polarity: Pos

I) Conditional ligand: GLS-Z-RL; Empirical formula: C38H57N11O9;

Exact Mass: 811.4341; Molecular Weight: 811.9275

Peak #: 1; Retention Time: 9.613 min; Base Peak m/z: 406.7002;

Base Peak Intensity: 3287573; Polarity: Pos

J) Conditional ligand: FAP-Z-AL; Empirical formula: C41H52N8O8;

Exact Mass: 784.39; Molecular Weight: 784.91

Peak #: 1; Retention Time: 9.693 min; Base Peak m/z: 785.3885;

Base Peak Intensity: 582117; Polarity: Pos

K) Conditional ligand: FAP-Z-KL; Empirical formula: C44H59N9O8;

Exact Mass: 841.45; Molecular Weight: 842.01

Peak #: 2; Retention Time: 9.253 min; Base Peak m/z: 421.7008;

Base Peak Intensity: 2788083; Polarity: Pos

FIG. 6. (figure S2)

Binding of Abc ligands to HLA-A*02:01 and H2-K^(b). Photocleavable ligands on a) A*02:01 and b) K^(b) molecules were peptide-exchanged with either previously identified peptide antigens (1, 9, 12 and 18) or Abc ligands (13 to 17, 19 and 20) following UV irradiation. Peptide ligands that can bind to the MHC will stabilize the MHC complex. ELISA was used to detect intact MHC molecules before UV irradiation (−UV), after UV irradiation in the absence (+UV) or in presence of binding peptides.

FIG. 7. (figure S3)

In vitro refolding and biotinylation of HLA-A*11:01, A*02:01 and H2-K^(b) containing Abc ligands (4, 17 and 20 respectively). a) The refolded MHC complexes were purified using S200 size exclusion chromatography. Fractions corresponding to approximately 45 kDa were collected as indicated in red. b) Gel shift SDS-PAGE was performed to assess the proportion of biotinylated MHC molecules. Purified MHC molecules yielded two distinct bands corresponding to the heavy chain (˜33 kDa) and beta2m (˜12 kDa). In the presence of soluble streptavidin, the biotinylated heavy chains bind to streptavidin forming complexes of high molecular size. IB:HC and IB: beta 2m refers to the heavy chain and beta2m extracted from E. coli inclusion bodies respectively.

FIG. 8. (figure S4)

Crystal structure of HLA-A*11:01 (grey) in complex with the Abc ligand, AIM-Z-YPK (cyan). a) Side view of the complex in cartoon format showing that the azobenzene moiety protrudes from the peptide binding cleft of the MHC. b) Top-down view in cartoon format showing the orientation that the Abc peptide resides in the binding cleft. c) Representation of the complex in its side surface view to show the depth of the ligand binding in the cleft. d) Top-down surface view shows that the ligand fits into the pockets of the MHC binding cleft. Z is the Abc of formula II.

FIG. 9. (figure S5)

Interactions between AIM-Z-YPK and residues in the binding groove of HLA-A*11:01. a) Top-down zoomed-in view of the HLA-A*11:01 peptide binding cleft. HLA-A*11:01 residues that contact the Abc ligand are highlighted as grey sticks. Electron density omit map (dark grey mesh) of the AIM-Z-YPK ligand (cyan). b) Interaction map depicting the contacts (represented by black dotted lines) made between the Abc ligand (amino acid in blue azobenzene moiety in red) and HLA-A*11:01 residues (black). Numbers representing bond distances are in Å. Z is the Abc of formula II.

FIG. 10. (figure S6)

Alternate conformations of the AIM-Z-YPK ligand. The normal “cis” binding confirmation of the Abc ligand (left) and the “trans” cross-linking conformation (right), of the two complexes in the asymmetric unit are shown. Z is the Abc of formula II.

FIG. 11. (figure S7)

HLA-A*11:01 molecules with AIM-Z-YPK ligand bound in alternate conformations give rise to two different molecular species. The canonical “cis” binding conformation of the ligand results in HLA-A*11:01 monomeric complexes and the noncanonical “trans” binding conformation results in HLA-A*11:01 dimeric complexes. a) The two species yielded two fractions in size exclusion chromatography. b) Repeated size exclusion chromatography with the separated fractions (fractions 1, left and 2, right) shows that the species did not interconvert in solution. c) Particle size of the HLA-A*11:01:AIM-Z-YPK species were determined using dynamic light scattering to be 171 Å and 98 Å in diameter for fraction 1 (left) and 2 (right) respectively. Z is the Abc of formula II.

FIG. 12. (figure S8)

Crystal structure of HLA-A*02:01 (grey) in complex with the Abc peptide, GLS-Z-RL (orange). a) The side view of the structure reveals that the Abc moiety of GLS-Z-RL ligand sits lower in the MHC peptide-binding cleft and is not as exposed as in the HLA-A*11:01 complex. b) 90 degree flip around the x-axis to show the top-down view in cartoon format of the peptide binding in the cleft. c) View of (a) in surface format. d) View of (b) in surface format. Z is the Abc of formula II.

FIG. 13. (figure S9)

Interactions between GLS-Z-RL and residues in the binding groove of HLA-A*02:01. a) Top-down zoomed-in view of the HLA-A*02:01 peptide binding cleft. Abc ligand-interacting residues of HLA-A*02:01 are highlighted as grey sticks. Electron density omit map (dark grey mesh) of the AIM-Z-YPK ligand (orange). The aromatic rings of the azobenzene moiety are not in the same plane due to a slight twist around the N═N bond. b) Interactions (represented by black dotted lines) made between the Abc ligand (amino acid in orange, azobenzene moiety in red) and HLA-A*02:01 residues (black) are shown in an interaction map. Z is the Abc of formula II.

FIG. 14. (figure S10)

MHC stability ELISA of sodium dithionite-mediated peptide-exchanged HLA-A*02:01 and H2-K^(b) molecules. a) A*02:01 refolded in vitro with 17 were peptide-exchanged with two A*02:01-restricted epitopes (12 and 21) and an A*11:01-restricted epitope (1) in the presence of 5 to 20 mM sodium dithionite. b) Similar to (a), refolded K^(b) molecules bearing 20 were peptide-exchange with two K^(b)-restricted epitopes (18 and 22) and an L^(d)-restricted epitope (9). Negative controls with no peptides added (−) to the MHC molecules were included.

FIG. 15. (figure S11)

Preferential cleavage of Abc ligands by sodium dithionite. a) A mixture containing 9 and 17 at 1:1 molar ratio (0.123 mM each) and varying concentrations of L-Glutathione oxidized (0 to 125 mM) were incubated in the absence (empty bars) or presence of 2.5 mM sodium dithionite (filled bars) for 5 minutes. b) Similar to (a), peptide mixture of 9 and 17 (0.123 mM each) were added to varying concentrations of L-Cystine (0 to 125 mM) prior to treatment with 2.5 mM sodium dithionite. Data are represented as the ratio of the intact Abc ligand 17 to dithionite-resistant 9 detected in LC-MS after sodium dithionite treatment.

FIG. 16. (figure S12)

Flow cytometric analysis on the viability of sodium dithionite-treated CD8⁺ T cells. Freshly isolated human PBMCs were incubated with sodium dithionite ranging from 1 mM to 100 mM to assess cellular toxicity of sodium dithionite. Cells were treated for 1 hour (top row), 1 hour followed by rested overnight in fresh culture media (middle row) or 16 hours (bottom row) prior to staining with anti-CD8 antibodies, Annexin V and LIVE/DEAD® viability dye. Data represented above are based on CD8⁺ cells. Numbers in each plot are expressed as a percentage of total CD8⁺ population.

FIG. 17. (figure S13)

Detection of antigen-specific CD8⁺ T cells using A*02:01 tetramers generated from UV-mediated peptide exchange or sodium dithionite-mediated peptide exchange. Freshly isolated peripheral blood mononuclear cells from an A*02:01-positive volunteer were stimulated with A*02:01 epitopes EBV LMP2426-434 (upper row) and CMV pp65495-503 (lower row). A*02:01-EBV LMP2426-434 (columns 1 and 4), A*02:01-CMV pp65495-503 (columns 2 and 5) and A*02:01-Abc (column 3) tetramers were used to perform tetramer staining 14 days post-in vitro stimulation. Number in each plot represents tetramer-positive cells expressed as a percentage of total CD8⁺ T cells. Plots in columns 1 to 2 and columns 3 to 5 are tetramer staining performed using UV-derived tetramers and Abc-derived tetramers respectively.

FIG. 18. (figure S14)

Detection of antigen-specific CD8⁺ T cells using H2-K^(b) MHC tetramers generated from UV-mediated peptide exchange or sodium dithionite-mediated peptide exchange. Freshly isolated splenocytes from naive and OTI-TCR transgenic C57/BL6 mice were mixed in 1:1 ratio. K^(b)-OVA₂₅₇₋₂₆₄ (columns 1 and 4), K^(b)-Tgd057₅₇₋₆₄ (columns 2 and 5) and K^(b)-Abc (column 3) tetramers were used to detect OT1 cells from the splenocyte mixtures. Numbers in each plot represent tetramer-negative (left) and tetramer-positive (right) CD8⁺ splenocytes expressed as a percentage of total splenocyte mix. Plots in columns 1 to 2 and columns 3 to 5 are tetramer staining performed using UV-derived tetramers and Abc-derived tetramers respectively.

FIG. 19.

Peptide binding of MBP₈₅₋₉₉ (yellow) to HLA-DR2 (blue) molecule. 15 residues (P-4 Glu to P11 Arg) of the MBP peptide were shown with peptide side chains of the P1 Val, P4 Phe, P6 Asn and P9 Thr occupying pockets within the peptide binding groove of the MHC molecule. (Smith et al. J. Exp. Med. 1998, 188, 1511-1520.) This figure is not part of the manuscript.

FIG. 20.

Crystal structure (left) versus model (right) of the HLA-DR2-MBP₈₅₋₉₉ complex. MBP₈₅₋₉₉ (yellow) binds to HLA-DR2 (blue) with the Anp residue at P4 position occupying the large hydrophobic P4 pocket of the MHC molecule. (Grotenbreg et al. J. Biol. Chem. 2007, 282, 21425-21436.) This figure is not part of the manuscript.

FIG. 21.

Abc-ligands for HLA-A*02:01.

FIG. 22.

Peptide exchange of known non-(CTELKLSDY and IVTDFSVIK); intermediate-(SLENFRAYV; ALQLLLEV and VMLRWGVLA) and high affinity binders (NLVPMVATV; GILGFVFTL, SLYNTVATL and NMLSTVLGV) to HLA-A*02:01 using HLA-A*02:01-ILKZGV (A) or HLA-A*02:01-ILKZKV (B) and Na2S2O4-induced peptide exchange (values are the mean±SD of two independent experiments) or HLA-A*02:01-KILGFVFJV and UV-induced peptide exchange (C). The presence of intact HLA complex was determined by MHC stability ELISA. The measured absorbances at 414 nm were evaluated relative to that of the high affinity binder NLV which was put to 100%

FIG. 23

Human peripheral blood cells (PBMC) were stained for the presence of antigen-specific T cell responses using PE-labeled tetramers. The flow cytometric results are depicted in the figure. The Abc and UV tetramers render similar results.

Staining of antigen-specific T cell responses against 4 different CD8 epitopes restricted to HLA-A*02:01 in a PBMC sample. Abc: PE-labeled tetramers generated using Abc ligand peptide exchange technology; HLA-A*02:01-ILKZGV. Abc*: PE-labeled tetramers generated using Abc ligand peptide exchange technology; HLA-A*02:01-ILKZKV. UV: PE-labeled tetramers generated using UV-induced peptide exchange technology; HLA-A*02:01-KILGFVFJV.

SI: Stain Index.

EXAMPLES Materials and Methods Abc Ligand and Antigenic Peptide Synthesis

The azobenzene-containing (Abc) MHC ligands were manually constructed by standard Fmoc-based solid-phase peptide synthesis. Fmoc-protected amino acids and Wang-based resins were purchased from Advanced ChemTech. The azobenzene linker was constructed as described (Verhelst et al., 2007). All other chemicals were purchased from Sigma-Aldrich. Deprotection and coupling of amino acids was carried out manually in a rotating glass reactor vessel at 0.2 mmol scale. For each peptide, the MBHA Resin HS, 100-200 mesh, 1% DVB (105 mg, 0.2 mmol, 1 equiv) was allowed to swell for 12 min in N-methyl-2-pyrolidinone (NMP). Installation of HMPB linker (120 mg, 0.5 mmol, 2.5 equiv) was accomplished using hydroxybenzotriazole (HOBT) (68 mg, 0.5 mmol, 2.5 equiv), benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate (PyBOP) (260 mg, 0.5 mmol, 2.5 equiv) and N,N-diisopropylethylamine (DIPEA) (246 ul, 1.5 mmol, 7.5 equiv) in 4 ml NMP. The HMPB-linked resin was washed for 12 min in NMP, followed by 12 min in dichloromethane (DCM). The first amino acid (0.8 mmol, 4 equiv) was coupled using N,N-diisopropylcarbodiimide (DIC) (124 ul, 0.8 mmol, 4 equiv), 4-dimethylaminopyridine (DMAP) (4 mg, 0.033 mmol, 0.165 equiv) in 4 ml DCM. The resin was then washed in DCM for 12 min, followed by 12 min in NMP. The amino acid/azobenzene linker was Fmoc-deprotected for 15 min using a solution of 20% piperidine in NMP. Following amino acid couplings were carried out using HOBT (108 mg, 0.8 mmol, 4 equiv), PyBOP (416 mg, 0.8 mmol, 4 equiv) and DIPEA (392 ul, 2.4 mmol, 12 equiv) in 4 ml NMP. Azobenzene linker (204 mg, 0.4 mmol, 2 equiv) coupling was carried out twice using PyBOP (208 mg, 0.4 mmol, 2 equiv) and DIPEA (196 ul, 1.2 mmol, 6 equiv) in 2 ml NMP. A Kaiser test (Kaiser et al., 1970) was used to monitor reaction completeness. Stepwise deprotection and coupling of the appropriate amino acids or azobenzene linker furnished the desired peptide on-resin. The peptides were cleaved, and simultaneously deprotected from dried resin using 5 ml trifluororoacetic acid (TFA) solution containing 2.5% distilled water and 2.5% triisopropyl silane (TIS) over 24 hrs. The peptide solution was precipitated in cold diethyl ether, and dried under vacuum. The peptide identities were confirmed by IT-TOF LC/MS analysis (Shimadzu).

Cleavage of Abc Ligands with Sodium Dithionite

Abc ligand and IPAAAGRFF were mixed at 1:1 molar ratio (0.123 mM each) and incubated in the presence of 1 mM, 2.5 mM or 5 mM freshly prepared sodium dithionite (in 200 mM phosphate buffer, pH 7.4). The reactions were allowed to proceed for 1 to 5 min until quenched using ZipTipC18 (Milipore) to extract the peptides from the sodium dithionite solution. The peptides were then eluted in 0.1% trifluoroacetic acid containing 5% acetonitrile and analyzed on IT-TOF LC/MS (Shimadzu).

In Vitro Folding and Purification of MHC Complexes

MHC molecules were generated as described previously (Garboczi et al., 1992). Genes encoding human β2-microglobulin and luminal portion of HLA-A*11:01, A*02:01 and H2-K^(b) engineered with a C-terminal BirA recognition sequence were cloned into pET-28a (+) vector (GenScript). The plasmids were transformed and overexpressed in E. coli BL21 induced by 1 mM isopropyl β-D-thiogalactopyranoside. The expressed proteins were extracted and purified from the inclusion bodies under reducing conditions and solubilized in 8 M urea. In vitro refolding of the MHC molecules was carried out with at least 10-fold molar excess of either UV-cleavable or Abc ligands for 24 to 36 h. The proteins were dialyzed into 20 mM Tris (pH8.0), biotinylated in vitro by recombinant BirA and purified using S200 size exclusion chromatography. Biotinylated MHC molecules were conjugated with Streptavidin-PE (Invitrogen) at 4:1 molar ratio to form MHC tetramers. For MHC molecules used in crystallography, refolding and purification were carried out in a similar fashion with the exception that unbiotinylated constructs were used. Also, the proteins purified from size exclusion chromatography were further subjected to ion exchange chromatography on a Mono Q column in 20 mM Tris (pH 8.0) and eluted over a gradient of increasing salt concentration with 20 mM Tris (pH 8.0), 1 M NaCl. For both HLA-A*11:01 and HLA-A*02:01, the proteins eluted at approximately 100-150 mM NaCl.

Peptide Exchange Conditions on MHC Monomers and Tetramers

MHC monomers used for MHC stability ELISA were peptide-exchanged in the presence of 100 fold molar excess of peptide ligands. For photocleavable MHC monomers, preparations of 500 nM MHC monomers in PBS were subjected to 365 nm longwave UV irradiation on ice for 15 minutes using UVP CL-1000 L Ultraviolet crosslinker (UVP), followed by the addition of 50 uM peptide ligands and 1-hour incubation on ice. For Abc MHC monomers, preparations containing 500 nM MHC monomers, 50 uM peptide ligands and 5 to 20 mM sodium dithionite in 50 mM HEPES (pH 7.4) were incubated for 30 minutes on ice. To stain antigen-specific CD8⁺ T cells, photocleavable MHC tetramers were diluted to 40 ug/ml with cold PBS containing 200 μM peptides, subjected to 365 nm longwave UV irradiation on ice for 15 minutes and followed by 1-hour incubation on ice. 40 μg/ml Abc MHC tetramers were incubated with 10 mM sodium dithionite in 50 mM HEPES (pH 7.4) containing 200 μM peptides and followed by 30-minute incubation on ice. After incubation, all MHC monomers and tetramers were further incubated for 1 hour at 37° C. with shaking at 850 rpm and were centrifuged at 16 000×g, 4° C. for 10 minutes prior to use.

MHC Stability ELISA

Assessment of ABC ligand binding to MHC molecules and optimization of ABC peptide exchange conditions were performed using an established protocol (Rodenko et al., 2006). Briefly, wells of a 384-well microplate (Corning) coated overnight at room temperature (RT) with 50 μl of 2 μg/ml streptavidin in PBS were washed and treated with 100 μl of 2% BSA in PBS for 30 min at RT. The 2% BSA was discarded and 25 μl of 20nM peptide-exchanged MHC was added to each wells and incubated on ice for 1 h. Wells were then washed and incubated with 25 μl 1 μg/ml HRP-conjugated anti-β2m antibodies (Clone D2E9, Abcam) on ice for 1 h. Subsequently, wells were washed and developed with 25 μl of ABTS solution (Invitrogen) for 10 to 15 min at RT. The development is quenched by the addition of 12.5 μl of 0.01% sodium azide in 0.1M citric acid. Absorbance was measured at 415 nm using Spectramax M2 microplate reader (Molecular Devices). Each washing procedure involves rinsing the wells four times with 100 μl of 0.05% Tween 20 in PBS. Samples were measured in quadruplicates.

Cells and MHC Tetramer Staining

Fresh whole blood was obtained from A*11:01 and A*02:01-positive volunteers. Isolation of PBMCs from these samples was performed via Ficoll-Paque density-gradient centrifugation. The isolated PBMCs were frozen for later staining without stimulation or were cultured in RPMI 1640 containing 2.05 mM L-glutamine (Invitrogen) supplemented with 40 μM 2-mercaptoethanol (Gibco), 100 IU/ml penicillin/streptomycin (Invitrogen) and 5% pooled human AB serum (Invitrogen) at 37° C., 5% CO₂. Briefly, PBMCs were stimulated with peptides at 10 μg/ml. 25 U/ml interleukin-2 (IL-2) (R&D systems) was added to the culture 2 days post peptide stimulation. Half medium change was carried out and 25 U/ml IL-2 was supplemented every 2 to 3 days from 5 to 14 days post stimulation.

Mouse splenocytes were extracted from spleens of naïve and OTI-TCR transgenic C57/BL6 mice using conventional splenocyte extraction protocol. Briefly, spleen meshed and homogenized in cold PBS was passed through a cell strainer. The resultant cells were washed with cold PBS and treated with 3 ml of RBC lysis buffer (pH 7.4) containing 155 mM NH₄Cl, 10 mM KHCO₃ and 0.1 mM EDTA for 2 min. Finally, the cells were washed twice with 10 ml of cold PBS and resuspended in 5 ml of cold PBS.

Cells were first stained with cell viability LIVE/DEAD® fixable near-IR stain (Molecular Probes®) prior to tetramer staining Subsequently, cells were washed with PBS and incubated with 80 nM peptide exchanged PE-conjugated MHC tetramers on ice for 20 min. Cells from A*11:01-positive donor were stained with 200 nM MHC tetramers instead. All PBMCs and murine splenocytes were stained with anti-human CD8 (Clone RPA-T8, BD biosciences) or anti-murine CD8 (Clone 53-6.7, BD biosciences) Pacific Blue™ antibodies for 15 min respectively. Cells were then washed again with PBS and fixed with 1% paraformaldehyde in PBS. Flow cytometry data were acquired on BD LSRII flow cytometer and analyzed using FlowJo (Tree Star).

Cell Viability Assay

10⁶ freshly isolated PBMCs from healthy volunteers were incubated in 1 ml RPMI 1640 culture media containing HEPES-buffered 1 mM to 100 mM sodium dithionite (pH 7.4) at 37° C., 5% CO₂. After 1-hour or 16-hour incubation, the cells were immediately assessed for cell viability or rested overnight in fresh media (for 1-hour treatment only). The cells were harvested, washed with PBS twice and stained with cell viability LIVE/DEAD® fixable near-IR stain (Molecular Probes®). The cells were then washed again with PBS and stained with anti-human CD8 (Clone RPA-T8; BioLegend) Brilliant Violet 421™ antibodies for 15 minutes. Thereafter, the cells were washed once with PBS and once with 1× Annexin V binding buffer (10 mM HEPES, pH 7.4; 140 mM NaCl; 2.5 mM CaCl₂) prior to incubation with Annexin V FITC (eBioscience) for 10 minutes. The stained cells were immediately analyzed on BD LSRII flow cytometer and data were processed using FlowJo (Tree Star).

Mass Spectrometry Analysis of Epitope Modification

50 uM of Influenza A MP₁₃₋₂₁ and EBV BMLF-1₂₅₉₋₂₆₇ peptides were incubated with 10 mM Na₂S₂O₄ in 50 mM HEPES (pH 7.4) or 0.3 mM NaIO₄ in PBS at RT for 2 h. After which, the peptides were extracted from the buffer using ZipTipC18 (Milipore) and loaded on LC/MS IT-TOF (Shimadzu) for analysis. 50 uM peptides in PBS were used as a control.

Competition Assay for Cleavage of Abc Ligands and Disulfide Bonds

GLS-Z-RL and IPAAAGRFF (0.123 mM each) were mixed with 2.5 mM, 25 mM or 125 mM L-Glutathione oxidized (Sigma-Aldrich) or L-Cystine (Sigma-Aldrich) and incubated with 2.5 mM freshly prepared sodium dithionite (in 200 mM phosphate buffer, pH7.4). After 5 minutes, the peptides were extracted from the L-Glutathione oxidized or L-Cystine, and sodium dithionite mixture using ZipTipC18 (Milipore). Elution of the peptides was carried out in 0.1% trifluoroacetic acid containing 5% acetonitrile prior to analysis on IT-TOF LC/MS (Shimadzu).

X-Ray Structures of HLA-A*11:01:AIM-Z-YPK and HLA-A*02:01:GLS-Z-RL Complexes

We performed X-ray crystallographic studies to determine the molecular details in which class I MHC molecules bind to the azobenzene-containing peptide.

Crystallization Conditions for HLA-A*11:01:AIM-Z-YPK and HLA-A*02:01:GLS-Z-RL Complexes

Crystals for HLA-A*11:01:AIM-Z-YPK were grown at room temperature using the sitting drop, vapor-diffusion method with a well solution of 15% (w/v) PEG4000, 0.2 M ammonium sulfate, 0.1 M tri-sodium citrate (pH 5.6). Crystals for HLA-A*02:01:GLS-Z-RL were grown at room temperature using the sitting drop method with a well solution of 20% (w/v) PEG4000, 10% (w/v) isopropanol, 0.1 M HEPES pH 7.5. Crystals were harvested and frozen rapidly in liquid nitrogen for data collection.

X-Ray Data Collection and Structure Refinement of HLA-A*11:01:AIM-Z-YPK and HLA-A*02:01:GLS-Z-RL Complexes

X-ray diffracted intensities for HLA-A*11:01:AIM-Z-YPK were collected at 100 K using a FRE generator at the Biopolis Shared Facilities, Singapore with a R-AXIS IV++ imaging plate detector from Rigaku. The data was collected at X-ray wavelength of 1.54 Å. X-ray data for HLA-A*02:01:GLS-Z-RL were collected at 100 K using the X06DA beamline (X-ray wavelength of 1.0 Å) at the Swiss Light Source with a Pilatus detector. Diffraction data (Table S3 for A*11:01 and Table S5 for A*02:01) for both HLA complexes were integrated with Mosflm and intensities were scaled with SCALA (Evans, 2006; Leslie, 1992). The structures were solved by molecular replacement in the program MOLREP (Vagin and Teplyakov, 2000), using the HLA-A*11:01 structure with PDB code 2HN7 (Blicher et al., 2006) or the HLA-A*02:01 structure with PDB code 3V5H, as search probe for HLA-A*11:01 and HLA-A*02:01 respectively. For HLA-A*11:01, refinement was carried out with REFMAC and BUSTER (Murshudov et al., 1997; Smart et al., 2012), with a final refinement was carried out on REFMAC. For HLA-A*02:01, the structure was refined initially with REFMAC, followed by final refinement rounds with Buster. Validation of the models and the x-ray data were checked with MOLPROBITY (Davis et al., 2007), and figures were generated using PyMOL (Delano, 2002). The coordinates and structure factors (code 4BEO for the HLA*A11:01 complex and 4BLH for the HLA*A02:01 complex) have been deposited in the Protein Data Bank.

The Crystal Structure of HLA-A*11:01:AIM-Z-YPK and HLA-A*02:01:GLS-Z-RL Complexes

We performed X-ray crystallographic studies to determine the molecular details of the interaction between class I MHC molecules and the azobenzene-containing peptide.

Overall Description

The X-ray structure of the HLA-A*11:01 molecule in complex with the azobenzene containing peptide was determined to 2.43 Å resolution (Table S3 and Fig S4). The model contains residues 1-274 of the heavy chain of HLA-A*11:01, residues 1-99 of β2-microglobulin and the azobenzene containing peptide, AIM-Z-YPK. There are two molecules in the asymmetric unit. The overall structure of the HLA-A*11:01/β2m/peptide complex is similar to the native peptide complex [PDB code 2HN7], and the RMSD for all Cα atoms of the alpha chain of the MHC molecules is 0.607 Å. The structure of the HLA-A*02:01 complex, which consists of residues 1-275 of the heavy chain, residues 1-100 of β32-microglobulin, and the azobenzene-containing peptide, GLS-Z-RL, was determined to 2.1 Å resolution (Table S5 and Fig S7). There are two molecules in the asymmetric unit. Superimposition of the HLA-A*11:01/β2m/peptide complex with the previously solved structure of HLA-A*02:01 structure [PDB code 3V5H] is similar overall, and the RMSD for all Cα atoms of the alpha chain of the MHC molecules is 1.11 Å.

REFERENCES CITED IN THE MATERIALS AND METHODS SECTION

-   Blicher, T., Kastrup, J. S., Pedersen, L. Ø., Buus, S., and     Gajhede, M. (2006). Structure of HLA-A*1101 in complex with a     hepatitis B peptide homologue. Acta Crystallogr. Sect. F Struct.     Biol. Cryst. Commun. 62, 1179-1184. -   Davis, I. W., Leaver-Fay, A., Chen, V. B., Block, J. N., Kapral, G.     J., Wang, X., Murray, L. W., Arendall, W. B., Snoeyink, J.,     Richardson, J. S., et al. (2007). MolProbity: all-atom contacts and     structure validation for proteins and nucleic acids. Nucleic Acids     Res. 35, W375-W383. -   Delano, W. L. (2002). The PyMOL Molecular Graphics System, DeLano     Scientific LLC. Palo Alto, Calif., USA. -   Evans, P. (2006). Scaling and assessment of data quality. Acta     Crystallogr. D Biol. Crystallogr. 62, 72-82. -   Garboczi, D. N., Hung, D. T., and Wiley, D. C. (1992).     HLA-A2-peptide complexes: refolding and crystallization of molecules     expressed in Escherichia coli and complexed with single antigenic     peptides. Proc. Natl. Acad. Sci. U.S.A. 89, 3429-3433. -   Kaiser, E., Colescott, R. L., Bossinger, C. D., and Cook, P. I.     (1970). Color test for detection of free terminal amino groups in     the solid-phase synthesis of peptides. Anal. Biochem. 34, 595-598. -   Leslie, W. A. G. (1992). Recent changes to the MOSFLM package for     processing film and image plate data. Joint CCP4+ESF-EAMCB     News-Letter on Protein Crystallography. -   Murshudov, G. N., Vagin, A. A., and Dodson, E. J. (1997). Refinement     of macromolecular structures by the maximum-likelihood method. Acta     Crystallogr. D Biol. Crystallogr. 53, 240-255. -   Rodenko, B., Toebes, M., Hadrup, S. R., van Esch, W. J. E.,     Molenaar, A. M., Schumacher, T. N. M., and Ovaa, H. (2006).     Generation of peptide-MHC class I complexes through UV-mediated     ligand exchange. Nat Protoc 1, 1120-1132. -   Smart, O. S., Womack, T. O., Flensburg, C., Keller, P., Paciorek,     W., Sharff, A., Vonrhein, C., and Bricogne, G. (2012). Exploiting     structure similarity in refinement: automated NCS and     target-structure restraints in BUSTER. Acta Crystallogr. D Biol.     Crystallogr. 68, 368-380. -   Vagin, A., and Teplyakov, A. (2000). An approach to multi-copy     search in molecular replacement. Acta Crystallogr. D Biol.     Crystallogr. 56, 1622-1624. -   Verhelst, S. H. L., Fonović, M., and Bogyo, M. (2007). A mild     chemically cleavable linker system for functional proteomic     applications. Angew. Chem. Int. Ed. Engl. 46, 1284-1286.

Results

We explored the application of azobenzene-containing (Abc, Z) linkers that are sensitive to sodium dithionite (Na₂S₂O₄). The recently developed stereocenter-free building block is accessible from readily available starting materials by a straightforward and cost-effective synthesis route. Furthermore, the Abc moiety is unaffected by reducing agents common to biological protocols (e.g. TCEP, DTT) and the correct fragmentation conditions have been demonstrated to be compatible with biomolecules and living systems.[5]

The Abc-linker, with its 12 bond lengths separating the amino- and carboxylic acid functionalities, cannot formally be regarded as tetrapeptide isostere (11 bond lengths, FIG. 1a ), but was envisaged to act as a surrogate for four amino acid residues, making allowances for the double bond and aromatic systems counting towards the peptidomimetic backbone. Conditional ligands were designed such that the Abc building block strategically replaced non-essential residues within a parent epitope of high affinity (Table S1 and S2), which improves the likelihood of the resulting Abc ligand to bind to and stabilize the recombinant MHC sufficiently during in vitro refolding and purification. For example, in the HLA-A*11:01-restricted epitope from hepatitis B virus DNA polymerase 110-118 (Table S1), the residues at positions P4-P7 are solvent-exposed, identifying them as candidates for Abc replacement.[6] Moreover, the key N- and C-terminal anchor residues Ile (P2) and Lys (P10) were conserved to ultimately furnish the Abc-homologue AIM-Z-YPK (4), which was obtained through standard Fmoc-based solid-phase peptide synthesis (SPPS) (Fig S1). Applying the same strategy, we obtained a panel of Abc ligands for HLA-A*11:01, HLA-A*02:01 and H2-Kb (Table S1, Fig S1); covering allelic variants of MHC predominantly found in Asian, and Caucasian populations, as well as in common murine disease models.

To determine whether the Abc-ligands' binding to the MHC product they were designed for was unperturbed by the tetrapeptide isostere, we started with a UV-sensitive complex, discharged its peptide cargo by traditional irradiation, and subsequently measured the capability of the Abc-ligand (which is inert to photocleavage) to prevent disintegration of the emptied complex by MHC stability ELISA (FIG. 1b for HLA-A*11:01, Fig S2 for HLA-A*02:01 and H2-Kb). As the protein requires all subunits to maintain a stable conformation, peptides that rescued the complex were deemed appropriate to produce purified Abc-ligand:MHC molecules (Fig S3). Definitive proof of peptide association, and the molecular details in which the Abc ligand binds the MHC, was furnished by X-ray crystallographic studies. The structure of HLA-A*11:01 in complex with AIM-Z-YPK (4) was determined to 2.43 Å resolution (Table S3, Fig S4). The conditional ligand (FIG. 1 d, cyan) engages the HLA in a way very similar to the parent peptide (FIG. 1 d, yellow) and occupies the peptide-binding groove by preserving crucial hydrogen bonds and salt bridges formed by the parent peptide via its N- and C-terminal anchor residues (Table S4, Fig S5). The central azobenzene moiety protrudes straight from the groove, is solvent exposed, and sufficiently straddles the four amino acids it was designed to replace. We furthermore observed 4 to occupy two alternative confirmations in the crystal (Fig S6). Optimization of the size exclusion protocol demonstrated that the two refolded A*11:01 complexes could be separately obtained (Fig S7a and b). These molecular species did not interconvert when left in solution, and had hydrodynamic volumes of 98 Å and 171 Å as judged by dynamic light scattering, that likely corresponding to the monomeric and dimeric MHCs respectively (Fig S7c). A second crystal structure of HLA-A*02:01 binding to GLS-Z-RL (17) at 2.1 Å resolution essentially displays the same features (but in this case no alternative conformation for the ligand is observed, Tables S5 and S6, Fig S8 and S9).

We next examined how to facilitate rapid and complete Abc-peptide exchange. Exposure of 4 to dithionite indeed resulted in fragmentation towards the expected two aniline products 7 and 8 as confirmed by LC/MS (FIG. 2a ). By mixing in a stable internal standard 9 at 1:1 ratio, and interrupting the reaction (ranging from of 1 to 5 mM of sodium dithionite) by solid phase extraction, the kinetics could be tracked by LC/MS analysis (FIG. 2b ). An incubation period of 5 min with 2.5 mM Na2S2O4 (aq) was sufficient for the original Abc ligand to fall below the limit of detection, indicative of (near) quantitative peptide cleavage in solution.

The peptide exchange efficiency (spanning 5 to 20 mM dithionite) was analyzed by ELISA on purified Abc-ligand:MHC complexes with established T cell epitopes (Table S2). Reduction-promoted peptide exchange could be observed at all tested dithionite concentrations (FIG. 2c for A*11:01, Fig S10 for A*02:01, and Kb). Disulfide bonds remain intact under these conditions (Fig S11) and the method therefore appears to have limited effect on the overall stability of the protein complexes. For HLA-A*11:01, the highest signal-to-background ratio was obtained at 20 mM Na2S2O4, yet this trend was the reverse for HLA-A*02:01, highlighting that every allelic variant carrying a tailored Abc-ligand will have unique stability characteristics.

A further impetus for moderating the amount of employed reducing agent is to prevent toxicity towards cells. It would be preferable that the MHC tetramers of novel specificity can be directly deployed, which involves them being shortly (<1 h) incubated with CD8+ T cells, without requiring the removal of any component (i.e. employed reagents or side-product) that could unnecessarily lengthen or complicate the peptide exchange and/or staining protocol. Both primary and cultured cells of various origins, fortunately, were very tolerant to buffered dithionite, showing little sign of apoptosis or cell death at high (10 mM) concentration and prolonged (16 h) exposure (FIG. 2 d, Fig S12). Balancing the above constraints, we employed 10 mM Na2S2O4 (aq) in the ensuing experiments.

To confirm that our strategy enables detection of antigen-specific cells from peripheral blood, a short-term expanded T cell line from an A*11:01-carrying donor responsive to Epstein Barr Virus (EBV) antigen (BRLF1134-142, 1, FIG. 3 top row) was labeled with MHC tetramers before, and after replacement with the canonical epitope (1), and an irrelevant peptide (10). This established that MHC tetramers generated through chemical- or UV-mediated exchange were equally capable in detecting frequencies of CD8+ T cells (i.e. 4.16% and 4.03%, respectively) only of the correct specificity and with minimal background. This could be replicated in an alternative CD8+ T cell line reactive towards Influenza A M113-21 peptide (10) when presented by A*11:01 (FIG. 3, bottom row), and reductive exchange of Abc-ligands was comparably successful for human HLA-A*02:01 and murine H2-Kb tetramers (FIGS. S13 and S14 respectively).

Next to preserving protein integrity, it is vital that cleavage conditions do not alter any functionality on the replacement epitope either. Such modifications could pose problems when they occur on critical residues that anchor the peptide to the MHC or are important for T cell receptor engagement, possibly resulting in failure to identify a given T cell population. A major limitation, for example, of vicinal diol- or alkanolamine-containing amino acids that can be cleaved by periodate, is that e concomitant oxidation of the Cys-, Met-, N-terminal Ser- or Thr-residues can be oxidised.[7] We therefore compared reductive (i.e. 10 mM dithionite) with oxidative (i.e. 0.3 mM periodate) cleavage conditions on well-established T cell epitopes containing said residues. Incubation with periodate predictably cleaved the N-terminal Ser of A*11:01-restricted Influenza A MP13-21 epitope (10), and (partially) oxidized the Cys and Met of EBV BMLF-1259-267 epitope (11), whereas dithionite treatment left the epitopes unaffected (FIG. 4).

Collectively, we have established a truly bio-orthogonal and robust strategy for conditional peptide exchange based on a unique panel of chemolabile Abc-ligands that can provide functional libraries of T cell labeling reagents both for human MHC molecules frequently found in both Asian and Caucasian populations, as well as for murine MHC. The true value of our method lies in the facile epitope replacement without the need for dedicated UV-irradiation equipment under conditions that are neither detrimental to the protein, the epitope, nor to the cells. Broad population coverage, through the inclusion of diverse MHC allelic variants, is currently under development, as we believe this will allow widespread application of this high-throughput method with which we can tackle the sprawling diversity of biologically relevant T cell populations in both basic research and clinical settings.

Alternate conformations of the Abc ligand in the HLA-A*11:01 complex We also observed electron density suggesting that the Abc ligand has an alternate conformation that is non-canonical to peptide-HLA binding; the C-terminal portion of the Abc ligand proceeding from the azobenzene group ‘flips out’ and binds the adjacent molecule in the asymmetric unit, forming what appears to be a ‘cross-link’ that would allow the two MHC molecules to dimerize (Fig S6). The occupancies of the canonical and non-canonical conformation were estimated to be 36% and 64% respectively. This was calculated based on their expected average B-factor values.

TABLE S1 Sequences of synthesized Abc ligands Peptide Parent epitope no. Abc ligand Restriction Sequence Organism Protein Location IEDB ID 3 AIF-Z-TK A*11:01 AIFQSSMTK Human Reverse 158 to 166  1913 immunodeficiency transcriptase virus 1 4 AIM-Z-YPK A*11:01 IMPARFYPK Hepatitis B virus DNA polymerase 110 to 118 27530 (2024) (peptide homologue) 5 QVPL-Z-YK A*11:01 QVPLRPMTYK Human Nef protein 73 to 82 52760 immunodeficiency virus 1 6 KTF-Z-PK A*11:01 KTFPPTEPK SARS coronavirus Nucleoprotein 362 to 370 33667 13 FLPS-Z-SV A*02:01 FLPSDFFPSV Hepatitis B virus Core protein 18 to 27 16833 14 LLF-Z-YV A*02:01 LLFGYPVYV Human Transcriptional 11 to 19 37257 T-lymphotropic activator Tax virus 1 15 NLVP-Z-TV A*02:01 NLVPMVATV Human herpesvirus 5 65 kDa lower 485 to 493 44920 matrix phosphoprotein 16 NLVP-Z-VATV A*02:01 NLVPMVATV Human herpesvirus 5 65 kDa lower 485 to 493 44920 matrix phosphoprotein 17 GLS-Z-RL A*02:01 GLSRYVARL Hepatitis B virus Polymerase 412 to 420 21145 19 FAP-Z-AL K^(b) and D^(b) FAPGNYPAL Sendai virus Nucleoprotein 324 to 332 15248 20 FAP-Z-KL K^(b) and D^(b) FAPGNYPAL Sendai virus Nucleoprotein 324 to 332 15248

Table S1: Alternate conformations of the Abc (Z) in the HLA-A*11:01 complex. We also observed electron density suggesting that the Abc ligand has an alternate conformation that is non-canonical to peptide-HLA binding; the C-terminal portion of the Abc ligand proceeding from the azobenzene group ‘flips out’ and binds the adjacent molecule in the asymmetric unit, forming what appears to be a ‘cross-link’ that would allow the two MHC molecules to dimerize (Fig S6). The occupancies of the canonical and non-canonical conformation were estimated to be 36% and 64% respectively. This was calculated based on their expected average B-factor values. The design of Abc ligands is based on the following parent epitopes. The restriction element, sequence, organism and protein source of the parent epitopes are listed. Residues in these epitopes that are replaced by the Abc moiety are underlined. IEDB ID refers to the epitope identification number in the immune epitope database and analysis resource (URL: http://www.immuneepitope.org/).

TABLE S2 Sequences of antigenic peptides Peptide no. Sequence Restriction Organism Protein Location IEDB ID 1 ATIGTAMYK A*11:01 Human Transcription activator 134 to 142 5002 herpesvirus 4 BRLF1 2 FLPSDFFPSV A*02:01 Hepatitis B virus Core protein 18 to 27 16833 10 SIIPSGPLK A*11:01 Influenza A virus Matrix protein 1 13 to 21 58567 11 GLCTLVAML A*02:01 Human BMLF1 protein 259 to 267 20788 herpesvirus 4 12 CLGGLLTMV A*02:01 Human Latent membrane 426 to 434 6568 herpesvirus 4 protein 2 21 NLVPMVATV A*02:01 Human 65 kDa lower matrix 485 to 493 44920 herpesvirus 5 phosphoprotein 18 SIINFEKL K^(b) Gallus gallus Ovalbumin 258 to 265 58560 22 SVLAFRRL K^(b) Toxoplasma Tgd057 57 to 64 146017 gondii 9 IPAAAGRFF L^(d) Toxoplasma Rhoptry protein ROP7 435 to 443 103992 gondii

Table S2: Previously identified antigenic peptides that were used in this study for MHC stability ELISA and generation of peptide-specific MHC tetramers are listed. IEDB ID refers to the epitope identification number in the immune epitope database and analysis resource (URL: http://www.immuneepitope.org/).

TABLE S3 Data collection and refinement statistics of HLA-A*11:01:AIM-Z-YPK Data collection Name HLA-A*11:01:AIM-Z-YPK Beamline Rigaku Detector R-AXIS IV++ Space group P1 Cell dimensions a, b, c (Å) 52.14, 71.46, 75.43 α, β, γ (°) 106.74, 96.74, 105.28 Resolution (Å) 29.93-2.43 (2.494-2.431)* R_(merge) (%) 5.2 (17) I/σ(I) 17.4 (6.8) Completeness (%) 94.7 (90.7) Redundancy 4 (4) Refinement Resolution (Å) 29.93-2.43 (2.49-2.43)* Number of reflections 33278 (2260) R_(work)/R_(free) 0.18/0.25 Number of atoms Protein 6252 Ligand 79 Water 249 B-factors (Å²) Protein 30.30 Ligand 23.73 Water 27.67 RMSD values Bond lengths (Å) 0.014 Bond angles (°) 1.693 Ramachandran values Most favoured (%) 96.3 Additional allowed (%) 3.7 Disallowed (%) 0.0 *Values for the highest resolution shell are shown

TABLE S4 Interactions between AIM-Z-YPK and HLA-A*11:01 Hydrogen-bond partner Abc-Peptide HLA Distance Van der Waals Residue Atom Residue Atom (Å) interactions Ala1 N Tyr7 OH 3.2 Met5, Tyr7, Glu63, N Tyr171 OH 2.9 Tyr195, Arg163, Tip167, O Tyr159 OH 2.6 Tyr171 Ile2 N Glu63 Oε1 2.8 Tyr7, Tyr9, Met45, Glu63, Asn66, Val67, Tyr99, Tyr159, Arg163 Met3 N Tyr99 OH 3.2 Asn66, Tyr99, Arg114, Tyr159 Tyr8 O Trp147 Nε1 2.9 Ala152 Pro9 Asp77 Lys10 N Asp77 Oδ1 3.0 Asp77, Thr80, Tyr84, Asp116, Thr143, Lys146 OXT Tyr84 OH 2.7 OXT Thr143 Oγ1 3.0 O Lys146 Nζ 3.0 Nζ Asp116 Oδ2 2.8 H-bond cut off <3.5 Å, Van der Waals: 3.6-4.0

TABLE S5 Data collection and refinement statistics of HLA-A*02:01:GLS-Z-RL Data collection Name HLA-A*02:01:GLS-Z-RL Beamline Swiss Light Source X06DA Detector Pilatus Space group P2₁ Cell dimensions a, b, c (Å) 57.79, 79.58, 83.97 α, β, γ (°) 90, 89.96, 90 Resolution (Å) 28.89-2.10 (2.21-2.10)* R_(merge) (%) 4.6 (9.4) I/σ(I) 14.0 (7.9) Completeness (%) 92.3 (81.3) Redundancy 2 (1.7) Refinement Resolution (Å) 28.89-2.10 (2.21-2.10) Number of reflections 43603 (2861) R_(work)/R_(free) 0.18/0.22 Number of atoms Protein 6217 Ligand 681 Water 537 B-factors (Å²) Protein 16.73 Ligand 16.59 Water 23.58 RMSD values Bond lengths (Å) 0.010 Bond angles (°) 1.04 Ramachandran values Most favoured (%) 98.4 Additional allowed (%) 1.6 Disallowed (%) 0.00 *Values for the highest resolution shell are shown

TABLE S6 Interactions between GLS-Z-RL and HLA-A*02:01 Hydrogen-bond partner Abc-Peptide HLA Distance Van der Waals Residue Atom Residue Atom (Å) interactions Gly1 N Tyr7 OH 2.6 Met5, Tyr7, Glu63, N Tyr171 OH 2.8 Tyr159, Trp167, O Tyr159 OH 2.7 Leu2 O Lys66 Nζ 2.9 Tyr7, Glu63, Lys66, Val67, Tyr99, His70, Tyr159 Ser3 Oγ Tyr99 OH 2.9 Lys66, His70, Tyr99, N Tyr99 OH 2.9 Tyr159 Abc O1 Tyr116 OH 3.5 Arg8 O Trp147 Nε1 2.8 Thr73, Val76 Leu9 N Asp77 Oδ1 3.1 Asp77, Leu81, Tyr116, OXT Tyr84 OH 2.8 Tyr123, Thr143, OXT Thr143 Oγ1 2.7 Trp147 H-bond cut off <3.5 Å, Van der Waals: 3.6-4.0

TABLE S7 Common human Crystal Reference MHC Class II Frequency structure Design of three (PubMed molecules of allele^(a)) (PDB ID#)^(b)) Parent ligand^(c)) Abc ligands^(e)) ID)^(e)) HLA-DR1 (DRA,  5% 2IAM GEL I GI L N A AK V PAD GELI-Abc-AAKVPAD 17334368 DRB1*01:01) GELIGI-Abc-KVPAD GELIGIL-Abc-VPAD HLA-DR2 (DRA, 13% 1YMM ENPV V HF F K N IV T PR ENPVV-Abc-NIVTPR 15821740 DRB1*1501) ENPVVH-Abc-IVTPR ENPVVHFF-Abc-TPR HLA-DR4 (DRA, 17% 3O6F FS W GA E G Q RP G FG FSW-Abc-QRPGFG 21297580 DRB1*04:01) FSWG-Abc-RPGFG FSWGAE-Abc-GFG HLA-DP2 4%, 6% 3LQZ RK F HY L P F LP S T RKF-Abc-FLPST 20356827 (DPA1*01:03, RKFH-Abc-LPST DPB1*02:01) RKFHYL-Abc-ST HLA-DQ8 23%, 43% 4GG6 QQYPSG Q GS F Q P SQ Q NPQ QQYPSGQ-Abc-PSQQNPQ 23063329 (DQA1*03:01, QQYPSGQG-Abc-SQQNPQ DQB1*03:02) QQYPSGQGSF-Abc-QNPQ Common mouse Crystal Reference MHC Class II structure Design of three (PubMed molecule N/A (PDB ID#)^(b)) Parent ligand Abe ligands^(c)) ID)^(d)) H-2-IAb — 3C5Z FE A QK A K A NK A VD FEA-Abc-ANKAVD 18308592 FEAQK-Abc-KAVD FEAQKA-Abc-AVD H-2-IAd — 1IAO I S QA V H A AH A EINEAGR IS-Abc-AAHAEINEAGR  9529149 ISQA-Abc-HAEINEAGR ISQAV-Abc-AEINEAGR H-2-IEk — 1KT2 ADL I AY L K Q AT K ADLI-Abc-QATK 11956295 ADLIA-Abc-ATK ADLIAYL-Abc-K ^(a))http://www.ncbi.nlm.nih.gov/projects/gv/mhc/ihwg.cgi?cmd=PRJOV&ID=9 ^(b))http://www.rcsb.org./pdb/home/home.do ^(c))P1, P4, P6 and P9 anchor residues are indicated by bold and underlined format ^(d))Structural design of Abc conditional ligands, MHC binding and fragmentation is achieved if the Abc moiety is incorporated betwe

 the critical P1 and P9 anchors, and replaces 4 amino acid residues. ^(e))http://www.ncbi.nlm.nih.gov/pubmed

indicates data missing or illegible when filed

CITED ART

-   [1] a) E. M. Sletten, C. R. Bertozzi, Angew. Chem. Int. Ed. Engl.     2009, 48, 6974-6998; b) M. Grammel, H. C. Hang, Nat. Chem. Biol.     2013, 9, 475-484. -   [2] a) G. Leriche, L. Chisholm, A. Wagner, Bioorg. Med. Chem. 2012,     20, 571-582; b) G. C. Rudolf, W. Heydenreuter, S. A. Sieber, Curr     Opin Chem Biol 2013, 17, 110-117. -   [3] a) J. D. Altman, P. A. H. Moss, P. J. R. Goulder, D. H.     Barouch, M. G. McHeyzer-Williams, J. I. Bell, A. J. McMichael, M. M.     Davis, Science 1996, 274, 94-96; b) M. M. Davis, J. D. Altman, E. W.     Newell, Nat. Rev. Immunol. 2011, 11, 551-558. -   [4] a) M. Toebes, M. Coccoris, A. Bins, B. Rodenko, R. Gomez, N. J.     Nieuwkoop, W. van de Kasteele, G. F. Rimmelzwaan, J. B. A. G.     Haanen, H. Ovaa, et al., Nat. Med 2006, 12, 246-251; b) G. M.     Grotenbreg, M. J. Nicholson, K. D. Fowler, K. Wilbuer, L.     Octavio, M. Yang, A. K. Chakraborty, H. L. Ploegh, K. W.     Wucherpfennig, J. Biol. Chem. 2007, 282, 21425-21436; c) A. H.     Bakker, R. Hoppes, C. Linnemann, M. Toebes, B. Rodenko, C. R.     Berkers, S. R. Hadrup, W. J. E. van Esch, M. H. M. Heemskerk, H.     Ovaa, et al., Proc. Natl. Acad. Sci. U.S.A. 2008, 105,     3825-3830; d) G. M. Grotenbreg, N. R. Roan, E. Guillen, R. Meijers,     J.-H. Wang, G. W. Bell, M. N. Starnbach, H. L. Ploegh, Proc. Natl.     Acad. Sci. U.S.A. 2008, 105, 3831-3836; e) E.-M. Frickel, N.     Sahoo, J. Hopp, M.-J. Gubbels, M. P. J. Craver, L. J. Knoll, H. L.     Ploegh, G. M. Grotenbreg, J. Infect. Dis. 2008, 198,     1625-1633; f) S. Gredmark-Russ, E. J. Cheung, M. K. Isaacson, H. L.     Ploegh, G. M. Grotenbreg, Journal of Virology 2008, 82,     12205-12212; g) C. X. L. Chang, A. T. Tan, M. Y. Or, K. Y.     Toh, P. Y. Lim, A. S. E. Chia, T. M. Froesig, K. D. Nadua, H.-L. J.     Oh, H. N. Leong, et al., Eur. J. Immunol. 2013, 43, 1109-1120. -   [5] a) S. H. L. Verhelst, M. Fonović, M. Bogyo, Angew. Chem. Int.     Ed. Engl. 2007, 46, 1284-1286; b) Y.-Y. Yang, M. Grammel, A. S.     Raghavan, G. Charron, H. C. Hang, Chemistry & Biology 2010, 17,     1212-1222; c) F. Landi, C. M. Johansson, D. J. Campopiano, A. N.     Hulme, Org. Biomol. Chem. 2010, 8, 56-59; d) G. Budin, M.     Moune-Dimala, G. Leriche, J.-M. Saliou, J. Papillon, S.     Sanglier-Cianférani, A. Van Dorsselaer, V. Lamour, L. Brino, A.     Wagner, Chembiochem 2010, 11, 2359-2361; e) G. Leriche, G. Budin, L.     Brino, A. Wagner, Eur J Org Chem 2010, 4360-4364. -   [6] a) J. Sidney, H. M. Grey, S. Southwood, E. Celis, P. A.     Wentworth, M. F. del Guercio, R. T. Kubo, R. W. Chesnut, A. Sette,     Hum. Immunol. 1996, 45, 79-93; b) T. Blicher, J. S. Kastrup, L. Ø.     Pedersen, S. Buus, M. Gajhede, Acta Crystallogr. Sect. F Struct.     Biol. Cryst. Commun. 2006, 62, 1179-1184. -   [7] a) B. Rodenko, M. Toebes, P. H. N. Celie, A. Perrakis, T. N. M.     Schumacher, H. Ovaa, J. Am. Chem. Soc. 2009, 131, 12305-12313; b) A.     Amore, K. Wals, E. Koekoek, R. Hoppes, M. Toebes, T. N. M.     Schumacher, B. Rodenko, H. Ovaa, Chembiochem 2013, 14, 123-131. 

1. A major histocompatibility complex (MHC) molecule comprising a ligand in the peptide binding groove of the MHC molecule, whereby said ligand comprises an azobenzene (Abc) wherein at least one of the aromatic rings comprises an electron-donor group and wherein the azobenzene comprises at least two amino acid residues separated by the azo-group of the Abc, and wherein the amino acid residues are positioned to interact with the peptide binding groove of the MHC molecule.
 2. The MHC molecule according to claim 1, wherein said ligand is an MHC peptide antigen of which amino acid residues that are located between the amino acid residues have been replaced by an Abc.
 3. The MHC molecule according to claim 1, wherein the Abc comprises the general formula I

wherein at least one of the aromatic rings comprise an electron-donating group; M is independently C, S, N or O; Z1 and Z4 each comprise an amino acid residue positioned to interact with the peptide binding groove of the MHC molecule; Z2 and Z3 are independently H, hydroxyl, carboxy, keto, or a linear or branched C₁-C₁₀ alkyl, optionally substituted with an oxy, hydroxyl, nitrogen, nitroxy, sulhydryl or sulfide group.
 4. The MHC molecule according to claim 1, wherein the Abc comprises the general formula II


5. The MHC molecule according to claim 1, wherein the ligand comprises the general formula III

wherein, A, B, C, D, X and Y are each independently an amino acid residue; n₁, n₂, n₃ and n₄ are each independently 0-11; and n₁+n₂+n₃+n₄ equals 2-18.
 6. The MHC molecule according to claim 1, wherein said Abc is a trans-Abc.
 7. The MHC molecule according to claim 5, wherein n₂ or n₃ or both are
 1. 8. A complex comprising at least two MHC molecules according to claim
 1. 9. A composition comprising an WIC molecule according to claim 1, and an MHC peptide antigen.
 10. A method of producing an MHC molecule comprising: producing an MHC molecule according to claim 1; contacting the produced MHC molecule with a reducing agent; and contacting said MHC molecule with an MHC peptide antigen.
 11. A method of detecting an MHC molecule comprising producing an MHC molecule according to the method of claim 10, and detecting the WIC molecule.
 12. A method according to claim 11, wherein the MHC molecule, a peptide in the peptide binding groove of the MHC molecule, or both, comprise a label.
 13. A solid surface comprising an MHC molecule according to claim
 1. 14. An azobenzene of formula I

wherein at least one of the aromatic rings comprise an electron-donating group; M is independently C, S, N or O; Z1 and Z4 each comprise an amino acid residue positioned to interact with the peptide binding groove of an MHC molecule; Z2 and Z3 are independently H, hydroxyl, carboxy, keto, or a linear or branched C₁-C₁₀ alkyl, optionally substituted with an oxy, hydroxyl, nitrogen, nitroxy, sulhydryl or sulfide group.
 15. An azobenzene according to claim 14, for use in the production of an MHC molecule comprising a peptide in the peptide binding groove of the MHC molecule. 